There are numerous applications in which it is preferable to transmit an analog or digital signal over an optical fiber instead of through an electrically conductive cable. An optical fiber offers several inherent advantages over a copper conductor, such as lighter weight and resistance to electromagnetic interference. In recent years, the cost of optical fiber systems has dramatically decreased, while their efficiency has proportionally improved, with losses typically less than 0.1 dB/km. Components, including diode-pumped lasers, modulators, and detectors are readily available from commercial sources to construct a system that can produce coherent light, modulate the light with an analog or digital signal, and convey the modulated signal some distance to a remote detector for demodulation to recover the signal.
However, the transmission of signals over optical fibers is not without its problems. For example, noise on a signal transmission system can significantly impact its ability to convey low amplitude signals developed by radio frequency (RF) remote antenna sources, due to the small optical modulation depth such signals typically develop. Noise affecting a modulated signal in a fiber optic system typically comprises three types, including thermal, shot, and intensity noise, any one or more of which can substantially degrade the system's signal-to-noise ratio (SNR). Aside from noise, several other problems with optical fibers systems have become more evident as low noise diode-pumped solid-state laser sources of increasing power have been developed. Although the higher power provided by such devices can help to improve the SNR signal distortion resulting from harmonic generation in the detector used to demodulate the light signal is a potential source of degraded performance in the system and potentially represents the primary disadvantage of higher power light sources. The problem arises because the higher power light produced by such sources develop high levels of photogenerated charge in the detector, which shield the electric fields comprising the optical signal from the applied bias voltage. If the light signal applied to the detector is sinusoidally modulated, shielding of the electric fields in the detector produces harmonics that distort its output signal.
Thermal damage to detectors can also occur when the power produced by the light source is sufficient to cause over heating. If high frequency photodetectors are required for a particular application, their size must be kept small to limit the effects of capacitance. However, a smaller photodetector is more susceptible to damaging thermal heating caused by receiving an optical signal from a high power laser source.
Higher power laser sources also tend to be more limited by Brillouin scattering, since this characteristic, which is associated with the propagation of an optical signal, increases exponentially with the product of optical power and the length of the optical fiber. Brillouin scattering is due to a coupling between acoustic waves in the optical fiber and the electromagnetic field associated with the optical signal; the scattered light produced by this effect propagates in the opposite direction from that of the optical signal. Since noise initiates the Brillouin scattering process, the SNR is deleteriously affected. Reducing the optical power minimizes the scattering process, but also reduces the SNR.
The conventional method for improving the SNR of an optically modulated signal is to increase the optical power of the signal or to increase the degree of modulation. For certain applications, it is not possible to increase the modulation depth, because the drive signal to the optical modulator is very weak, or because a higher modulation signal would drive the modulator into a non-linear region. The alternative is then to use higher power laser sources; yet, as noted above, increases in optical power are limited by the onset of signal distortion caused by harmonic generation in the detector, Brillouin scattering, or by the maximum photodetector current that might damage the detector. Thus, even though higher optical power sources are more readily available, the power of such sources is not usable in conventional systems. Accordingly, it is evident that there is a need for a new method of handling modulated light signals and for a system that uses higher powered optical sources to convey weak modulation depth signals at acceptable SNR levels not limited by detector distortion, possible detector damage, or Brillouin scattering.